Adding an extra magnet

[ding]

One of the links on the Linkwitz lab sites describes (in swedish which I cannot read) the addition of a second magnet to the rear of a driver with as far as I can tell no other changes to the magnetic circuit. I thought about the theory for a while then tested it (it didn't seem to make any difference) and discarded the idea as a waste of a good magnet, that is until I saw a slightly rearranged though fundametally identical design in an off the shelf morel woofer (The picture is on the inside back cover of Vance Dickason's loudspeaker cookbook 5th ed).

Does anyone have any ideas on how the second magnet interacts with the first to increase the gap magnetic field strength?
What do you suppose would be the effect on TS parameters?

Thanks

Dan

This is the site I mentioned

http://www.svalander.se/anlag/anlag.htm

[woody]

I think you are referring to adding a bucking magnet. The main goal of adding this magnet is to reduce the stray magnetic field.
In this a second magnet usualy about 80% as large as the primary magnet is added with the polarity reversed. That is at
an inch or so from each other they should repell each other then
when they do touch there is only a little magnetic attraction.
This is talked about over on the single driver forum. It seems
that there isn't a lot of change in the Thiel /Small parameters
maby a DB or so increse in sensitivity.

[planet10]

In theory a magnet added to the back in the appropriate orientation and with something to shunt the field should lower the drivers Qt, increase the efficiency, and make the magnetic field less "obtrusive" (ie shielded). There was an article in Speaker Builder on the subject (he used a tuna can to shunt the field).

I cooresponded with a fellow who tried it on some RS 40-1197s. He extracted a half dB of efficiency and IIRC no real changes in the T/S.

I have some experiments awaiting my auto-impedance measuring jog.

dave

[ding]

In theory a magnet added to the back in the appropriate orientation and with something to shunt the field

Dave,

What exactly do you mean by something to shunt the field? Is this some sought of iron circuit to carry the flux from the bucking magnet?

Are there any benefits other than the slight increase in efficiency and shielding?

Has anyone seen the Morel add in the back of the loudspeaker cookbook that I spoke of in my first post, I am sure this double magnet arrangement is not intended only for shielding, then again it could be a marketing ploy on their part.

Thanks

Dan

[planet10]

Originally posted by ding
What exactly do you mean by something to shunt the field? Is this some sought of iron circuit to carry the flux from the bucking magnet?

Yes, exactly.

dave

[Dan_Gleebles]

Just thought you might like this bit of info I was sent about bucking magnets

(Bare "bucking" magnets don't work that well. If correctly matched in
relative size to the main magnet (magnet simulation software required), they
reduce the field in some directions but may increase it in others. It also
depends on whether the chassis is steel or not.

If the speakers must be close to the TV then fully shielded drivers with a
complete steel can over a double magnet assembly are the best solution.

Two drivers back to back, with the magnets very close, may reduce the
external field and would be quite easy to try out.

Hope this helps.

regards,

Enrico Cecconi M.Sc.
Systems Engineer (Audio)
Visteon (UK) Ltd.)

[planet10]

Originally posted by Dan_Gleebles
Two drivers back to back, with the magnets very close, may reduce the
external field and would be quite easy to try out.

With the Radio Shack 40-1197 (whch i had in hand when i decided to test this) a two driver assembly magnet-to-magnet is effectively a shielded driver.

dave

[Larin]

If you stillare interested all he gives as a reason is that itDetta gav större exakthet åt återgivningen. Dessutom ökade känsligheten något.)=This gave a more exact reproduction(whatever that may mean) and in addition to this gave a slight increase in efficiency.

[Ilianh]

For dipole applications, one could benefit from this as it seems to increase Qts, atleast on fostex drivers, look at the differences between Fe series and FeAV series.

And a slight increase efficiency is always a good thing..